This invention relates to high intensity discharge lamps and more particularly to metal halide lamps with high color rendering indexes (CRI).
There are several metal halide lamp designs that yield color rendering indexes greater than 80 while maintaining a low color temperature of approximately 3100 K. One such design utilizes quartz as an arc discharge vessel material and includes iodides of sodium, scandium, lithium, dysprosium, and thallium within the chamber. The arc discharge vessel has a small volume, relative to conventional lamps, to provide enough arc discharge vessel wall temperature to sufficiently vaporize the additives and achieve the desired light output. This lamp design provides excellent light output properties; however, it does so only through a relatively short period of time. After approximately 2,000 hours of lamp operation the lamp color temperature decreases approximately 200 K and the lumen output also significantly decreases. Eventually, the lamp""s photometric properties degrade to the point where the light output is no longer acceptable since the color temperature decrease continues at a rate of approximately 100 K per 1,000 hours.
This high CRI metal halide lamp produces significant line emission and the primary factor contributing to the decrease in color temperature through life is the loss of the scandium line emission. The scandium emission decreases at a faster rate than the other elemental emission and as a result blue radiation is preferentially lost and the resulting color temperature decreases. A secondary factor contributing to the decrease in color temperature is the changing color of the quartz wall. Wall reactions take place between the added chemistry and the quartz wall leading to quartz devitrification. The devitrified zone changes in color from clear to yellow. This area then absorbs shorter wavelength light resulting in a further decrease in the color temperature of the lamp. A tertiary factor contributing to decreased color temperature during lamp life is an increase in arc discharge vessel wall temperature caused by a loss in quartz transmission. Once the devitrified zone forms, it begins to discolor and absorb light causing a rise in the arc discharge vessel wall temperature. Arc discharge vessel wall blackening caused by tungsten transport from the electrode during lamp operation also significantly reduces the transmission through the quartz and increases the arc discharge vessel wall temperature. The arc discharge vessel wall temperature increase causes an increase in the melt vapor pressure. The sodium pressure, which provides more yellow and red radiation through spectral line broadening, increases the greatest. The increased sodium pressure causes the color temperature to decrease further.
Many sodium-scandium metal halide systems utilize a scandium metal chip within the arc discharge vessel chamber as a getter for impurities. By gettering initial and lifetime impurities the required starting voltage remains low and the free-iodine content of the chemistry also remains low. This creates a system that starts easily and has acceptable initial photometric performance. The addition of the chip has one large draw back in that it reacts with the wall and the iodides and creates a stain on the inner surface of the arc discharge vessel wall. This stain absorbs light and in turn creates a hot spot within the arc discharge vessel. The stain can increase the wall temperature by 100xc2x0 C. This is not a concern in conventional metal halide lamps because the larger volume of the arc discharge vessel keeps the wall temperatures normally well below unacceptable wall temperatures. The conventional arc discharge vessels, which have volumes greater than 1 cc, operate at approximately 800xc2x0 C., so the chip addition, with its concomitant stain, may cause the arc discharge vessel temperature to reach 900xc2x0 C. This is an acceptable temperature for a long life lamp design having the larger arc vessel volume, considering the stain does disappear with time and the arc discharge vessel blackening becomes a more dominant factor in heating the arc discharge vessel.
Since the high CRI arc discharge vessel temperatures are approximately 940xc2x0 C. without the addition of scandium metal to the system it was to be expected that the arc discharge vessel temperature would increase to over 1040xc2x0 C. if scandium metal were used. Even if the stain disappeared during the early part of lamp life, the arc discharge vessel could not sustain these temperatures and the lamp life would be short caused by arc discharge vessel failure. The internal pressure of the arc discharge vessel during lamp operation is several atmospheres and this would cause the arc discharge vessel wall to bulge as the quartz viscosity decreased to a weakened state.
A second problem that was believed to be associated with the addition of scandium metal to the high CRI chemistry is that the metal halides are known to react with the chip during the initial aging of the lamp and thereby increase the scandium radiation. The increased scandium radiation would result in an unacceptably high color temperature because of the increased blue radiation. The color temperature would be increased to approximately 3500 K when the most desirable color is at 3100 K. Normally, this would be compensated for by reducing the amount of scandium iodide in the melt, however, in this particular chemistry, this would result in a very low scandium iodide amount in the arc discharge vessel and was believed that this would lead to even more dramatic color shifting when the scandium reacted with the wall during the life of the lamp. Another conventional method of reducing the initial color temperature is to increase the reflector coating height or thickness on the arc discharge vessel. Again, this technique would prove to be a problem because it would further increase the arc discharge vessel wall temperature and result in early arc discharge vessel failures.
To compensate for the lack of scandium metal in the small volume arc discharge lamp design, several clean processing steps were used to eliminate the contamination from the arc discharge vessel chamber during manufacture. These processes did reduce the initial contamination, reduce the starting voltage requirements, and produce a lamp with acceptable initial photometric performance, however, in spite of these improvements, the resulting lamp design still showed a steady color temperature decrease through the lamp life.
It is, therefore, an object of the invention to obviate the disadvantages of the prior art.
It is yet another object of the invention to provide a high CRI lamp with constant color through life.
These objects are accomplished, in one aspect of the invention, by the provision of an arc discharge lamp having a CRI greater than 80 and a given color temperature which exhibits a variance of  less than 3% over 2000 hours of operation, said lamp comprising: an arc discharge vessel of quartz having an electrode in each end, said vessel having a volume of about 0.25 to 0.8 cc, an iodide fill of 10 to 16 mg/cc, an amount of mercury in the range of 10 to 16 mg/cc, and an amount of scandium metal to maintain said given color temperature within said variance.